1. Field of the Invention
The present invention relates to a sensor system for ascertaining a torque between an input shaft and an output shaft which are connected to each other via a torsion bar.
2. Description of the Related Art
For this purpose, the sensor system includes a magnetic unit having a magnetic pole wheel for generating a magnetic field, the magnetic unit being rigidly connected to one of the two shafts; a flux ring unit having a flux ring system for converting and conducting the magnetic flux generated by the magnetic unit, the flux ring unit being rigidly connected to the other shaft; and a sensor unit having at least one first magnetic field-sensitive sensor element for sensing changes in the magnetic flux through the flux ring system, the sensor unit being mounted in a stationary manner in relation to the flux ring unit.
A sensor system of this type is described in published German patent application document DE 10 2005 031 086 A1. Its structure and the interaction between the individual components are illustrated by the exploded view in FIG. 1, in which neither the two shafts nor the torsion bar are represented for reasons of clarity. Magnetic pole wheel 11 of magnetic unit 1 is connected to one of the two shafts and situated concentrically to the shaft axis. It includes an alternating system of ring section-shaped north pole and south pole magnetic elements 12 and 13.
Flux ring system 20 of flux ring unit 2 includes an inner flux ring 21 and an outer flux ring 22 made of a ferromagnetic material. These two flux rings 21, 22 are connected to the other shaft, so that they are oriented concentrically to each other and to the shaft axis, i.e., parallel to magnetic pole wheel 11. Inner flux ring 21 is provided with teeth 23 which face outward from the shaft in the radial direction, while outer flux ring 22 is provided with teeth 24 which face inward toward the shaft in the radial direction. The two flux rings 21, 22 are situated and designed in such a way that their teeth 23 and 24 engage with each other alternatively and are spaced the same axial distance apart from magnetic pole wheel 11. The geometry of teeth 23, 24 as well as the distance between teeth 23, 24 are adapted to the ring section shape and the size of magnetic pole elements 12 and 13.
A sensor unit 3 having a magnetic field-sensitive sensor element 30 forms the third component of the known sensor system. This sensor unit 3 is mounted in a stationary manner in relation to flux ring system 20. It includes two magnetic flux-collecting elements 31, 32 which are used to selectively supply the magnet flux through flux ring system 20 to sensor element 30. For this purpose, magnetic flux-collecting elements 31 and 32 are situated adjacent to each other, one on the circumference of inner flux ring 21 and one on the circumference of outer flux ring 22. Sensor element 30 is located between magnetic flux-collecting elements 31 and 32, which are provided in this area with special formations 33 for sensor element 30.
If a torque is now generated between the input shaft and the output shaft, the torsion bar is twisted and magnetic pole wheel 11 is twisted against flux ring system 20. This produces a change in magnetic field strength at the location of sensor element 30. This change in magnetic field strength is converted to an electrical measuring signal with the aid of sensor element 30.
In the automotive industry, torque sensors of the type discussed herein are used, for example, in connection with steering systems. The torque-measuring signal is used in this case as a manipulated variable for an electrical power steering system. In this connection, it proves to be advantageous if the torque sensor is additionally equipped with an index function so that it detects the position of the steering wheel or steering column for driving straight ahead. This corresponds to a specified angle position of the input shaft or the output shaft, namely the 0° angle position.
Torque sensor systems having an index function of this type are already used in practice. Thus, methods are known for implementing the index function with the aid of mechanical switching elements, for example microswitches or gears.
Furthermore, a sensor concept is presented which provides a contactless implementation of the index function in the form of a magnetically operated switching element. This switching element is usually a digital Hall IC which is situated on a printed circuit board within the sensor unit. This digital Hall IC is switched with the aid of an index magnet which is attached to a defined position on the circumference of the flux ring unit. When the index magnet rotates past the sensor unit, the digital Hall IC emits a switching signal which indicates that the steering wheel is located in the position defined by the index magnet.
The contactless variant proves to be advantageous, compared to the mechanical implementation of the index function, since the index magnet and the switching IC are not placed under any mechanical load and are therefore also not subject to wear. However, the structure of the known sensor system having a magnetically implemented index function is relatively complex, in particular where the position of the index magnet and the mounting of the switching IC are concerned. The index magnet in this case is namely situated on the flux ring unit in such a way that it has minimal influence on the magnetic flux produced by the magnetic pole wheel in the flux rings. Due to this arrangement, the digital switching IC must be situated at a relatively great distance from the sensor element used to sense the changes in the magnetic flux through the flux rings. In practice, the switching IC is therefore mounted on the bottom of a printed circuit board, with the aid of a special support, while the magnetic field-sensitive sensor element is situated on the front of the printed circuit board.